1. Field of the Invention
This invention relates generally to polyphase induction electrical machinery systems and more specifically, to a device and method for synthesizing a desired output alternating current (xe2x80x9cACxe2x80x9d) necessary to operate a polyphase AC induction motor.
2. Description of Related Art
A polyphase AC induction motor is operated by an inverter drive system, as disclosed in U.S. Pat. No. 6,054,837 by the current inventor, and herein incorporated by reference in its entirety. The improvement over the prior art is that the number of independently driven phases is increased to more than the conventional three, preferably to a number substantially greater than three, such as twelve or more phases. The stator is wound with little or no chording, and with little or no winding distribution, both allowing windings with fewer turns to be used. Thus, resistance losses in the stator windings are reduced. Large machines with low pole counts are facilitated by the reduced winding distribution, again enhancing efficiency because low pole count machines are more efficient. Motor control configuration may be changed electronically to control the stator magnetic field structure.
The use of many phases reduces substantially the problems associated with harmonic oscillating fields. Specifically, the use of many phases causes harmonic fields, up to a number equal to the number of phases, to move in synchrony with the fundamental oscillating field. Both spatial and temporal harmonic fields are still developed, but such fields add beneficially to the fundamental oscillating field of the machine. Harmonics of higher order than the number of phases still excite non-synchronous oscillating fields; however, such high order harmonics are, in general, very weak. Therefore, motor efficiency losses associated with harmonic fields are reduced.
There are several sources of both spatial and temporal harmonics, including motor saturation effects and aspects of inverter design. A particular source of non-synchronous oscillating fields is a pulse-width modulation (xe2x80x9cPWMxe2x80x9d) carrier used by the inverter. The PWM carrier is the basic pulsing waveform produced by the inverter switching elements. The PWM carrier is modulated by changing the relationship between positive and negative pulses in order to produce the desired output waveform. In general, the higher the frequency of the PWM carrier, the greater the fidelity of the desired output waveform. However, this increase in fidelity comes at a price, including increased electromagnetic interference, increased switching element noise, and increased system cost. While not necessarily a harmonic of the desired output waveform, in that the frequency of the PWM carrier need not be a multiple of the frequency of the desired output waveform, the PWM carrier is capable of producing a non-synchronous field. Thus, there is a need to either reduce the intensity, or obtain benefit from, the oscillating field produced by the PWM carrier.
From the foregoing, it may be appreciated that there is a need for a method and device for synthesizing a desired output alternating current using pulse width modulation, in which the pulsing distortion produced by the pulse width modulation does not degrade the efficiency of the motor. In one embodiment of the present invention, this pulsing distortion is constrained by the selection of proper pulse width modulation techniques to produce a rotating magnetic field in the motor that is synchronous with the desired rotating magnetic field. A n phase order inverter drives a n phase order induction motor. The pulse width modulation is selected so as to produce a fixed number M of PWM pulses per output cycle, with M constrained to be less than n.
The pulse width modulation may be produced by well known techniques, for example through the use of half bridge or full bridge switching elements, driven by a logic controller.
It is an object of the present invention to produce a PWM oscillating field that is synchronous with the fundamental.
A technical advantage of the present invention is an increased efficiency.
Another technical advantage is an increased motor output power due to the increased efficiency
Another technical advantage is a decreased PWM switching frequency.
Another technical advantage is greater inverter efficiency due to the decreased PWM switching frequency.
Other technical advantages of the present invention are set forth in or will be apparent from drawings and the description of the invention that follows, or may be learned from the practice of the invention.